Generally, in a twin-screw kneading extruder, composite resin material like a plastic compound is manufactured by feeding macromolecular resin pellets constituting base material and a powdered additive into a barrel and transferring the pellets and the additive downstream while kneading the same by means of a pair of kneading screws passed through the barrel. This kind of twin-screw kneading extruder employs a mechanism in which motive power of a driving motor is divided into two parts by a speed regulator and the divided parts of the motive power are transmitted to the individual kneading screws.
Rotational resistance exerted on the kneading screws when the kneading screws are rotated is not constant. Since this rotational resistance varies with a kneaded material type, the number of fins formed on each kneading screw and rotating speed thereof, fluctuations in load occur in the kneading screws due to changes in the rotational resistance.
The frequency of this kind of load fluctuations is correlated with the rotating speed of the kneading screws. Thus, the higher the rotating speed of the kneading screws, the higher the frequency of the load fluctuations. Particularly because there is a strong need for enhancing productivity in recent years, the twin-screw kneading extruder is often operated while rotating the kneading screws at a high rotating speed or varying the rotating speed of the kneading screws using an inverter control technique, for instance. Therefore, if the frequency of the load happens to coincide with the natural frequency of torsional motion of a rotary shaft system made up of the screws of the twin-screw kneading extruder and a shaft of the speed regulator, resonance can occur, causing an increased risk of breakage of kneading screw shafts or the speed regulator.
Means generally used for avoiding the occurrence of this kind of resonance is to vary the natural frequency. For example, Patent Document 1 cited below proposes such means that a torsion bar or a coupling mechanism is provided on a drive shaft of a driving motor and the natural frequency of a rotary shaft system of a twin-screw kneading extruder is varied by regulating stiffness of a portion by which a driving force is transmitted from the driving motor to a kneading screw side.
Inverter-driven, variable-speed-type twin-screw kneading extruders increasingly used in recent years, however, do not operate at a constant kneading screw rotating speed. Therefore, even if the natural frequency of the twin-screw kneading extruder is varied, there is still a high risk of the occurrence of resonance depending on operating conditions and it has not been possible to prevent the occurrence of resonance in a reliable fashion.    Patent Document 1: Japanese Unexamined Patent Publication No. 1999-138528